Making lightningcss optional peer dependency

[RobinTail]

Consider a Node.js project using vitest for testing.

There is absolutely no CSS, but vitest requires vite and it (even v8.0.0-beta.0) nevertheless requires lightningcss as a runtime dependency, that also installs detect-libc and 11 binary packages named lightningcss-[OS].

I made an experiment by overriding it in my pnpm config:

overrides:
  "lightningcss": "npm:[email protected]"

And worked well, so I presume that this package is not really used when no CSS present.

Therefore, for vite v8 I propose moving lightningcss to optional peer dependencies, so that consumers can install it only when needed.